Pressure-sensitive conductive elastomer compound

This is a continuation of application Ser. No. 519,320, filed May 4, 1990, now abandoned, which is a continuation of application Ser. No. 220,389, filed Jul. 18, 1988, now abandoned, which is a continuation of application Ser. No. 906,759, filed Sep. 11, 1986, now abandoned.

This invention relates to pressure-sensitive conductive elastomer compounds and, more specifically, to a pressure-sensitive conductive elastomer compound of the type which exhibits high resistance (insulating performance) when it is in non-pressed condition, and of which the resistance, as the compound is pressed, will vary according to the magnitude of the pressure.

Hitherto, pressure-conductive materials have been known which are in the form of a conductive compound comprising a resilient material, such as rubber or the like, and a conductive filler mixed therewith. For such filler, metallic particles, such as nickel, conductive carbon black, graphite particles and the like are normally used. Such conductive compound, molded into a rod or sheet form, is widely used today as a switching element, or as a pressure-sensitive element for sensors such as pressure sensor and tactile sensor.

Conductive compounds of such conventional type have the following difficulties. Those incorporating metallic particles as a conductive filler are liable to change of properties with time due to oxidation of the particles; therefore, they lack stability and are often subject to chattering and noise generation. Those incorporating powdery masses of a conductive carbon black as a conductive filler provide only insignificant change in resistance when they are under pressure, because the particle diameter of the carbon black is extremely small, i.e., 20˜30 mμ; as such, they are of no practical use. If a granulated material formed of a conductive carbon black is used as such a filler, it is possible to provide greater variations in resistance, but a conductive compound incorporating such material is liable to particle breakage when it is under pressure; naturally, therefore, such compound lacks both durability and stability.

Where graphite particles are used as a conductive filler, no characteristic stability can be provided if they are of non-uniform shape as those of natural graphite. Therefore, it is known to use artificial graphite particles which have been rounded and freed of sharpness by pulverization, milling or otherwise to provide good characteristic stability.

Conductive compounds incorporating artificial graphite particles of such type are advantageous in that they are characteristically stable, durable, and less liable to noise generation, but on the other hand they have drawbacks in that preparation of graphite particles to the desired configuration requires a complicated and troublesome procedure and in that the attainable yield thereof is rather small.

This invention, made in view of aforesaid difficulties with the prior-art compounds, has as its primary object the provision of a pressure-sensitive conductive elastomer compound having highly stable conductive characteristics under pressure and which is easy to manufacture.

Another object of the invention is to provide a pressure-sensitive conductive elastomer compound whose conductive characteristics under pressure may be varied without changing the mechanical properties of the compound.

In order to accomplish the above and other objects, the compound according to the invention comprises a matrix material having insulating and elastomeric properties, and conductive particles of a macromolecular material having a spherical particle configuration and baked and carbonized, the conductive particles being incorporated and dispersed into the matrix material.

Materials available for use as aforesaid matrix material having insulating and elastomeric properties include natural rubber, synthetic rubbers, such as chloroprene rubber, SBR, NBR, and silicone rubber, thermoplastic elastomers, such as polyurethane, polyester, and EVA, and liquid rubbers, such as polyurethane and silicone. Particularly preferable among them is silicone rubber, a material having high heat resistance, excellent electrical properties, and good resistance to chemicals.

Macromolecular materials having a spherical particle configuration useful for the purpose of the invention include styrene, vinyl-chloride, vinylidene-chloride, methyl methacrylate, and furfuryl alcohol, all in spherical particle form prepared by suspension polymerization, and resol resins chemically pulverized in spherical particle form. The term "suspension polymerization" referred to herein means a process such that a polymerization catalyst is added to monomers, the mixture being stirred in water added with a dispersant to allow the monomers to disperse in oil-drop form, being then polymerized. The term "chemically pulverized" herein means that a resin dissolved in a solvent is cooled or added with a precipitant so that the resin is separated out in fine powder form.

The particle diameter of said conductive particles is 30˜120 μm, preferably 50˜100 μm, and the proportion of the particles to the compound as a whole is 20˜60% by volume. If the particle diameter is less than 30 μm, the possible variation in resistance of the compound is unreasonably small, while if it is greater than 120 μm, the particles cannot satisfactorily be dispersed in the matrix material. The proportion of the particles may be suitably determined according to the desired characteristics and sensitivity, and also to the type of the matrix material.

However, if it is less than 20% by volume, the compound may not exhibit any sufficient conductivity, and if it is more than 60% by volume, the variation in conductivity (resistance) when the compound is under pressure, from the conductivity level when the compound is not under pressure, is insignificant, the compound being thus of no practical use. Therefore, the proportion of the particles should be within the range of 20 vol. % to 60 vol. %.

According to the invention, spherical particles of a macromolecular material have conductivity given to them by being baked and wholy or partially carbonized. This facilitates the selection of particle size for the conductive particles. Therefore, it is possible to use particles having a uniform particle size, and thus to allow the compound to have highly stable pressure-sensitive conductive properties. Furthermore, the compound is easy to manufacture.

The degree of carbonization of the particles (thickness of the carbonized portion of each particle's spherical shell) can be varied by changing the degree of baking of the particles, and thus various conductivity grades of particles can be easily produced. Therefore, it is possible to provide varied pressure-sensitive conductive characteristics without changing the mechanical properties of the compound.

FIGS. 1a to 1c, inclusive, are sectional illustrations showing various degrees of carbonization of spherical particles of a macromolecular material. In FIG. 1a, only the surface area of a particle is carbonized. In FIG. 1b, nearly the entire portion of a particle is carbonized. In FIG. 1c, a particle is entirely carbonized.

FIG. 2 is a graph showing the pressing force-resistance relationships in Example 1.

FIG. 3 is a graph showing the pressing force-resistance relationships in Examples 2 and 3, in which graph the character (a) represents such relationships in Example 2 and (b) represents those in Example 3.

The compound according to the invention will now be described in detail with reference to the accompanying drawings.

FIGS. 1a through 1c are schematic views showing a few examples of spherical carbonized particles used for the purpose of the invention. Various conductivity grades of particles are shown as they are formed of non-conductive spherical particles of a macromolecular material. Experiments have revealed that the electric conductivity of the particles varies according to the heating and baking conditions. This is considered to be attributable to the following facts. If, as in FIG. 1a, only a region in the vicinity of the outer periphery of a particle 1 is carbonized thicknesswise (t) in a spherical shell pattern, the conductivity of the particle 1 is small because the carbonized portion 2, i.e., the portion having electric conductivity, is of a small volume. If carbonization progresses further to the extent that a larger part of the particle 1 is carbonized, as FIG. 1b shows, the conductivity of the particle 1 becomes considerably greater. Finally, if carbonization progresses still further until the particle is completely carbonized, the conductivity of the particle is maximized. Thus, even if particles 1 of same diameter are used, the carbonization degree of the particles varies according to the baking conditions applied. These facts are considered to be responsible for the variations in conductivity. Shown by 3 is a non-carbonized portion.

The degree of carbonization of particle 1 is adjustable by changes in baking conditions, such as heating temperature and time. Therefore, by baking and carbonizing preselected particles 1 having a specified diameter under preset baking conditions it is possible to easily obtain particles 1 having the required conductivity.

Particular examples are given hereinbelow to further illustrate the invention.